The present invention relates to an antenna which has a bandwidth as broad as 0.5 to 13 GHz, for instance, but is small in size and, more particularly, relates to an antenna using a semicircular radiator or semicircular, ribbon-shaped radiator.
In R. M. Taylor, "A Broadband Omnidirectional Antenna," IEEE AP-S International Symposium, 1994, p1294, there is disclosed a conventional broadband antenna using semicircular conductor discs as depicted in FIG. 1. This conventional antenna has two elements. One of the elements is composed of two semicircular conductor discs 12.sub.1a and 12.sub.2a, which have a common center line Ox passing through the vertexes of their semicircular arcs and cross at right angles. The other element is also composed of two elements 12.sub.1b and 12.sub.2b, which similarly have a common center line Ox passing through the vertexes of their semicircular arcs and cross at right angles. The two elements are assembled with the vertexes of their circular arcs opposed to each other. A feeding section is provided between the vertexes of the arcs of the two elements; a coaxial cable 31 for feeding is disposed along the center of one of the two elements, with the outer conductor of the cable held in contact with the element.
FIG. 2 illustrates a simplified version of the antenna depicted in FIG. 1, which has semicircular conductor discs 12a and 12b disposed with the vertexes of their semicircular arcs opposed to each other. The feeding section is provided between the vertexes of the two conductor discs 12a and 12b to feed them with the coaxial cable 31 installed in the conductor disc 12b.
FIG. 3 shows the VSWR characteristic of the antenna depicted in FIG. 2. It will be seen from FIG. 3 that the simplified antenna also has a broadband characteristic, which was obtained when the radius r of each of the semicircular conductor discs 12a and 12b was chosen to be 6 cm. The lower limit band with VSWR&lt;2.0 is 600 MHz. Since the wavelength .lambda. of the lower limit frequency in this instance is approximately 50 cm, it is seen that the radius r needs to be about (1/8).lambda.. The radiation characteristic of the antenna shown in FIG. 1 is non-directional in a plane perpendicular to the center line Ox, whereas the radiation characteristic of the antenna of FIG. 2 is non-directional in a frequency region from the lower limit frequency to a frequency substantially twice higher and is highly directive in the same direction as the radiator 12a in the plane perpendicular to the center line Ox.
Thus, the conventional antenna of FIG. 1 comprises upper and lower pairs of antenna elements each formed by two sectorial radiators crossing each other, and hence it occupies much space. Also in the simplified antenna of FIG. 2, the sectorial semicircular radiators are space-consuming. In terms of size, too, the conventional antennas require semicircular conductor discs whose radii are at least around 1/8 of the lowest resonance wavelength; even the simplified antenna requires a 2r by 2r or (1/4).lambda. by (1/4).lambda. antenna area. Accordingly, the conventional antennas have defects that they are bulky and space-consuming and that when the lower limit frequency is lowered, they become bulky in inverse proportion to it.